For the purpose of splitting the optical power available on a single fiber into several outputs, fiber optic couplers or splitters have been developed. Among the several types of splitters, some are made by fusing two or more optical fibers together by heating and drawing the fibers while they are held in mutual contact in the fusion region. The result of this method is a tapered region of fused optical material wherein coupling of optical power occurs among the fused fibers in a coupling region within the fused region. Optical fibers used for fabrication of splitters typically have outside cladding diameters of 40 microns (um) or more. While work has been done toward developing splitters made of three or more fused fibers, the most highly developed technology in this field relates to fused splitters composed of two fibers. These are referred to as two-port splitters or 2.times.2 splitters. The terms splitter and coupler are synonymous.
In order to obtain splitters with four, eight, twelve, sixteen, or more outputs, several two-port splitters have been arranged in a tree-like fashion. In this manner, e.g., a single input may be split into four outputs using three two-port splitters. Using the two-port splitter as a building block in tree structures suffers from several disadvantages including package size, fabrication cost, and long-term reliability.
Fused couplers have been made from seven fibers, wherein six fibers are arranged around a seventh inner fiber. The geometry of circles shows that six identical fibers can be perfectly arrayed around an identical central fiber so that each of the surrounding fibers so disposed will contact both of its neighboring fibers and the central fiber This is an ideal case from the viewpoint of fused splitter fabrication. The mutual contact among fibers promotes uniform fusion and a resultant relatively high probability that each of the seven outputs can be made to give 1/7th of the output optical power. Such a structure also can be made so that all the optical power input in the central fiber is equally coupled to the six surrounding fibers providing six outputs each giving about 1/6th of the output power.
Splitters using fewer than six surrounding fibers have been very difficult to make. The spacing of the surrounding fibers around the central fiber should be azimuthally periodic in order to obtain uniform outputs. The act of aligning and holding the surrounding fibers prior to and during fusion requires extreme precision. Moreover, the surrounding fibers cannot be held in place by their neighbors as in the seven fiber geometry. On the other hand, by prior methods, there has appeared to be no way to arrange more than seven fibers in a manner that can achieve equal power splitting.
Most distribution system requirements specify splitting in multiples of four. There has been no completely satisfactory means to fabricate a unitary, i.e., single optical structure, splitter that naturally provides four, eight, twelve, or sixteen outputs. The invention addresses that need.